1. Field of the Invention
The present invention generally relates to optically-pumped lasers, and more particularly, to optically-pumped lasers that have a low pump energy absorption per unit length such as may result from a low pump absorption cross-section and/or a low doping concentration of pump absorptive ions.
2. Description of Related Art
An optically-pumped laser includes a gain medium and an optical pump source that supplies optical pump radiation to the gain medium, where it is converted into a laser emission. Many early optically-pumped lasers utilized high intensity arc lamps that were formed into any suitable shape, such as a linear shape or a helically wrapped configuration. Although these sources emit high intensity light, they are xe2x80x9clow radiancexe2x80x9d (i.e. they emit over a very large solid angle), and therefore efficiency was greatly improved by using pump cavities to collect and redirect the pump light to illuminate the laser medium. Accordingly, the laser medium generally was configured into a long, the cylinder with a large side surface area, and its laser axis length was oriented to match the length of the arc lamp that pumped it. Early pump cavities utilized highly reflecting surfaces surrounding both the arc lamp and the laser medium, which collected and redirected the pump light through the side of the laser medium in a multi-pass configuration. Many versions of these so-called xe2x80x9cside pumpedxe2x80x9d or xe2x80x9ctransverse-pumpedxe2x80x9d optical pump cavity geometries were developed and used in laser products.
However, side-pumped laser configurations have many problems, such as conflicting requirements for cooling the laser medium through the medium side surfaces, suppressing parasitic pump cavity oscillations, and controlling the spatial distribution of the optical pump power within the gain medium, while still maintaining a high pump energy absorption efficiency. These problems are particularly difficult when the side-pumped laser medium has a low absorption of pump energy per transverse pass. In order to obtain high pumping efficiency in such low absorption media, the pump energy must be retained by the pump cavity and redirected back through the laser medium many times. When the pump energy losses resulting from the successive interactions with the cooling interface, parasitic suppression, and pump cavity optical systems become excessive, efficient optical pumping of the side-pumped laser medium cannot be achieved and high power laser operation may become impossible.
In more recent times high radiance pump sources such as lasers and diode laser arrays have been developed and utilized as pump sources for many laser media. Because the light from a high radiance source is emitted over a much smaller solid angle than from an extended lamp source, a high radiance pump source can be optically configured into a narrow beam by an optical system. In xe2x80x9cend-pumpedxe2x80x9d or xe2x80x9clongitudinal-pumpedxe2x80x9d configurations, the beam is introduced into the laser medium through one end and then travels along the laser axis down the length of the gain medium together with the laser emission. In some mode matched embodiments of end pumped lasers, the transverse optical pump radiation profile is matched to the desired transverse mode profile of the laser. In embodiments in which most of the pump energy is absorbed during transit along the round trip length of the laser medium (which is usually much larger than the two-pass transverse width of the laser medium), a pump cavity may not be required to attain high pumping efficiency.
However, significant problems render high power operation difficult to achieve in end-pumped configurations. For example, excessive optical power intensities are created due to the fact that end-pumped lasers have a common propagation axis for both the pump and extracted laser beams, combined with the fact that a typical laser diode pump beam has a highly non-uniform transverse intensity distribution. Attempts to design an efficient, practical high-power end-pumped laser have encountered problems such as excessive optical power intensity due to the combination of the intensities of the pump and extracted laser beams, severe thermally-induced medium distortion, excessive doping concentration or medium length constraints on design optimization, increased laser resonator optical losses resulting from complex multi-wavelength optical coatings, and spatially non-uniform pumping distributions. These problems are particularly severe for embodiments in which the laser medium exhibits a high pump saturation flux such as may result from a low pump absorption cross-section.
In addition to these problems, which can limit the performance and increase the complexity of end-pumped laser embodiments, an additional problem arises for an important class of laser media. Specifically, Òthree-levelÓ or Òquasi-three levelÓ laser media exhibit substantial performance benefits when the product of the medium dopant concentration and the active volume is minimized, i.e., when the dopant concentration is low by current standards. This concentration dependence arises due to the requirement that a substantial upper laser level population density must be maintained to overcome the equilibrium lower laser level population density. One such example of a concentration dependent laser material is ytterbium-doped yttrium aluminum garnet (ÒYb:YAGO), which has been identified to have potential for use in high power lasers. In order to take advantage of such concentration-sensitive media, end-pumped configurations utilizing diode lasers as the pump source have been proposed. However, such end-pumped configurations have encountered the above-discussed problems, which have restricted scaling them to high average power levels.
In order to quantify and compare the performance limitations of the prior art side pumping and end pumping concepts applied to concentration sensitive laser materials and to provide one measure of performance improvement over prior art which is afforded by the utilization of the present invention, it may be useful to define a concentration figure of merit, Fc, for the laser medium, which is obtained by dividing the minimum optical pump radiation focal spot area, Ai, by the product of the medium laser axis end area, A, the medium dopant concentration, No, the pump absorption cross-section, "sgr"p, and the effective end-pumping medium length, L. Symbolically:
Fc=Af/(ANo"sgr"pL).xe2x80x83xe2x80x83Eq. 1 
As used herein, the value of Fc provides a quantitative measure of the highest efficiency and lowest waste energy that can be realized from a given configuration of a concentration-sensitive medium. Generally, a higher Fc indicates that the laser is more efficient, while a lower Fc indicates that the laser is less efficient.
Prior to the conception and reduction to practice of the present invention, after consideration of all prior art shortcomings, it is believed that end-pumped configurations have provided the most efficient and scaleable configurations of concentration-sensitive media lasers such as Yb:YAG. In order to maintain a near optimum level of pump energy absorption in such configurations, the quantity No"sgr"pL in the above equation must be approximately equal to 1. Since the minimum value for A is equal to Af, it is apparent that for end-pumping embodiments the largest possible value of the concentration figure of merit is equal to 1, which requires that the optical pump radiation completely fill the medium volume. Such optimum end-pumped configurations can be impractical to implement, and generally must be compromised to a lower Fc value. Furthermore, side-pumped embodiments that use external re-entrant pump cavities have a low pump energy absorption efficiency and/or an Fc value significantly lower than 1.0.
In order to overcome the limitations of the prior art, the present invention provides an optically-pumped laser having a gain medium configured to provide a low loss, three-dimensional integrated optical pump cavity for injected optical pump radiation. The optical pump cavity re-directs the optical pump radiation throughout the lasing volume in multiple passes, substantially retaining the optical pump radiation within the lasing volume to create an average pump absorption length that can be configured to be much longer than twice the laser axis length, thereby efficiently extracting energy from the optical pump radiation even when very low pump absorptive ion dopant concentration laser media are employed. The integrated pump cavity can be implemented in lasers having a wide range of power levels, from low power lasers to high power lasers that generate one kW or more. Laser embodiments implementing the present invention have been demonstrated to have a high efficiency in converting the input pump energy into laser emission while maintaining uniform pumping distribution. Particularly, some laser embodiments have a concentration Figure of Merit (Fc) greater than 1.0, and in some embodiments exceeding 2.0.
An optically-pumped laser apparatus described herein comprises a gain medium that defines an integrated pump cavity having a plurality of boundaries contiguous with said gain medium that are reflective of the pump radiation, and an optical system that defines a laser axis through a first end and a second end of the gain medium, which may also be reflective to the pump radiation. An optical pump source supplies the optical pump radiation, and a beam delivery system is arranged to inject the optical pump radiation through one or more pump cavity windows into the gain medium with a propagation direction that is substantially not collinear with the laser axis. In some embodiments, the boundaries of the integrated pump cavity include all boundaries of the laser medium including those situated transverse to said laser axis, and the integrated pump cavity boundaries are configured so that injected optical pump radiation reflects between them while projecting longitudinally along the laser axis. The optical pump radiation is substantially contained and absorbed by the laser medium within the integrated pump cavity, thereby energizing said gain medium to generate a laser emission along said laser axis.
Many such integrated medium/pump cavity configurations can be implemented. In some embodiments, the optical pump cavity is designed to concentrate the optical pump radiation and uniformly and efficiently pump the entire volume of the lasing medium without significantly compromising the attainable beam quality of the laser beam generated within the lasing volume. In one such embodiment, the pump cavity includes converging surfaces that longitudinally concentrate the optical pump radiation as it projects along the laser axis.
In some embodiments, the gain medium comprises a solid state gain medium that defines a pump cavity including coated boundaries, thereby integrating the pump cavity with the gain medium. The integrated pump cavity is particularly useful for high power solid state lasers. By carefully designing the pump cavity configuration and appropriately choosing the dopant concentration of the active pump ion within the solid state material, high power optical pump radiation can be absorbed approximately evenly throughout the volume, producing heat more uniformly throughout the solid state laser gain medium, which can reduce or substantially eliminate higher order thermal distortion effects that adversely affect other high power optically-pumped lasers. Also, uniform absorption reduces or substantially eliminates potentially destructive hot areas within the gain medium. One embodiment includes a lightly doped (e.g.  less than 1%) solid state laser gain medium such as Yb:YAG, and the integrated pump cavity includes design features to concentrate the intensity of the optical pump radiation. When pumped to optical transparency conditions, the lightly-doped, quasi-three level solid state laser gain medium produces less heat and lower spontaneous emission losses per unit volume than the other, more highly doped crystals that are used in conventional laser designs.
Advantageously, a heat sink can be directly coupled to the transverse surfaces of the solid state gain medium, effectively cooling the gain medium and optical coating formed thereon, even for high power operation. Furthermore, detrimental higher order optical distortion effects are reduced because the product of the transverse temperature gradient and the laser axis medium length is minimized through the use of the integrated pump cavity. Also, because the optical pump radiation propagates substantially orthogonal to the laser axis, coating requirements for the end surfaces are simplified and the total intensity proximate to the laser end surfaces is significantly reduced, thereby reducing cost, increasing performance, and improving manufacturing yield.
The foregoing, together with other objects, features and advantages of this invention, will become more apparent when referring to the following specification, claims and the accompanying drawings.